The present invention relates to a spectrum spread transmission, and more particularly to spectrum spread transmission method and apparatus which allow simultaneous reception of a plurality of channels of information codes at close positions even when spread codes (PN codes or Pseudo Random Noise codes) having a short code length are used.
FIG. 10 shows a circuit configuration of a transmitter of a prior art transmission apparatus of a spectrum spread system and FIG. 11 shows a circuit configuration of a receiver.
Those circuit configurations are for a transmission apparatus which uses both a delayed detection binary phase shift keying modulation (DBPSK) system and a spectrum spread (SS) system.
A delayed code processing unit 2 converts an information code to be transmitted into a code sequence suitable for the delayed detection. An SS spread unit 3 conducts direct spread modulation of the spectrum spread system, a PN code generator 4 generates a spread code (PN code) to be used in the direct spread modulation and a PN clock generator 5 generates a clock pulse for defining a bit frequency f.sub.pn of the PN code. Hereinafter, for simplicity, the frequency f.sub.pn is referred to as a chip rate or a chip frequency.
The information code inputted from an input terminal 1 of the transmitter is code-converted by the delayed code processing unit 2, product-modulated by the spread code from the PN code generator 4 by the SS spread unit 3, and converted to a base band spread signal.
An LPF 6 is a low pass filter (LPF) having characteristics 11 diagrammatically shown in FIG. 12 by a chain line and it extracts a main robe 12 (shown by solid thick line) of a primary component from frequency characteristic components (shown by solid line) of the spread signal.
The main robe component of the spread signal extracted by the LPF 6 is modulated into a binary phase shift keying (BPSK) signal having a carrier frequency f.sub.h by the modulator 7 and it is transmitted from an antenna 8 as a transmission signal.
On the other hand, in the receiver of FIG. 11, a received signal from an antenna 20 is inputted to a BPF 21. The BPF 21 is band pass filter (BPF) which passes components having a band width of 2.times.f.sub.pn centered at the carrier frequency of f.sub.h and it extracts the main robe component of the transmitted modulated signal.
The extracted main robe signal is inverse-spread by multiplying the PN code by an SS inverse-spread unit 22, demodulated to a base band by a demodulator 23 of the BPSK system and converted to a digital signal by an A/D converter 24.
A process for the delayed detection is conducted by a delayed detection processing unit 25 which outputs it as an information code. The PN code generator 26 and the PN clock generator 27 have the same functions as those of the PN code generator 4 and the PN clock generator 5 of the transmitter.
For the PN code, it is necessary to use a code having a large difference between in-phase and out auto-correlation values. As a code having such a property, an m-channel pseudo noise code or Gold code having the number of repetition bits (hereinafter referred to as a chip length) represented by 2.sup.k -1 (where k is a positive number) has been known, and this code is usually used as the PN code.
The SS system has two major features. One is that a large spread process gain is attainable. For example, when a PN code having a chip length of 255 chips, a spread process gain of 10.times.log(255)=24 dB is attained.
This means that a transmission distance can be extended by ten times or more (which is comparable to a performance when a C/N of the received signal is increased by 24 dB) to compare with the transmission by only the DBPSK system. The larger the chip length of the PN code, the larger the spread process gain.
Thus, the PN code having as long a chip length as possible is usually used within a range permitted by the transmission band width and the transmission rate. C represents a carrier and N represents a noise.
The second feature is that one transmission band may be shared by a plurality of channels as diagrammatically shown in FIG. 13A. In the SS system, a signal of another channel which uses a different PN code is introduced into the received signal as a random noise, and it merely lowers the C/N of the received signal but does not disturb the SS inverse-spread process of that channel. Thus, in the transmitter of the SS system, a plurality of different PN codes having a small mutual correlation are usually used and the simultaneous transmission of the plurality of channels is conducted while compensating the reduction of the C/N by the spread process gain.
In the transmitter which uses such an SS system, a frequency band which is so-called ISM band (Industrial, Scientific and Medical Band) having a band width of 26 MHz from 2471 to 2497 MHz is allocated. The chip rate f.sub.pn of the PN code used in this band is usually set to not higher than 26 MHz/2=13 Mcps to permit the transmission of the main robe of the spread signal.
As a prior art wireless transmitter which uses the ISM band, a wireless LAN having a transmission rate of 256 Kbps has been known. While a detailed specification of the wireless LAN is not known because it has not been laid open, it is generally considered as follows according to a view of the inventors of the present invention. In this wireless LAN, a maximum usable chip length of the PN code is 13 Mcps/256 Kbps=50 chips. However, the 50-chip code which may be used as the PN code has not been publicly known. It may be possible to search by using a computer but it is not practical because the chip length is too long and the calculation amount is extremely large.
Accordingly, it appears that in this wireless LAN, the 31-chip channel code which is the channel code of the maximum usable chip length is used as the PN code.
Accordingly, the chip rate is 256 Kbps.times.31 chips=8 Mcps and the band width of the main robe of the transmission signal is double of that, that is, 16 MHz.
This value is smaller than the band width of 26 MHz of the ISM band. On the other hand, the code of the 31 chips or smaller number of chips has a small mutual correlation and includes a small number of highly independent codes. For example, for the 31-chip m-channel code, the number types of codes is only three. Thus, it appears that by providing two channels having different carrier frequencies (to allow the use of the same PN code in the respective channels) while utilizing a margin for the band width, the number of channels which may be simultaneously used is doubled.
In a system for preventing disasters or a monitoring apparatus, there is a strong demand for a wireless transmission apparatus which allows the transmission of motion pictures or semi-motion pictures and which does not need application or procedures for permission by the authorities. Further, as shown in FIG. 14, there is a strong demand for simultaneously receiving motion pictures or semi-motion pictures of two or more channels from two different points (usually, a plurality of points) at a center (base station).
A frequency band which can satisfy the first demand is the ISM band. On the other hand, in order to transmit the motion pictures, the transmission rate of at least four times of 256 Kbps or 1 Mbps, preferably 1.5 Mbps or larger is required even if the modern image compression technique is fully used. Further, a code error rate of at most one error per minute (the code error rate is not larger than 1/10.sup.8 for 1.5 Mbps) is required.
In the ISM band, it is obliged by the Radio Regulatory Law to use PN code having a chip length of not less than ten chips. Thus, in the SS system transmitter using the DBPSK, the transmission from 1 Mbps to 1.3 Mbps=13 Mcps/10 chips is permitted but the information code of 1.5 Mbps which is beyond the above limit cannot be transmitted. In order to transmit the information code having the transmission rate of 1.3 Mbps, it is necessary to replace the modulator 7 of FIG. 10 and the demodulator 23 of FIG. 11 from the binary phase shift keying (BPSK) system to the quadrature phase shift keying (QPSK) system to increase the transmission rate.
Circuit configurations modified to the QPSK system are shown in FIGS. 15 and 16. Those circuits are devised by the inventors of the present invention. In the QPSK system, an input information code is separated into two components I and Q, which are modulated by a sine wave and a cosine wave of a carrier for transmission.
An SS spread unit 3 of FIG. 15 ss-spreads the I and Q components and it is different from the spread unit 3 of FIG. 1 in the internal circuit configuration. However, both function in the same way except that the same process as the SS spread in the BPSK is applied to the two components I and Q. Thus, the same reference numerals are used for the components of the circuit and the SS spread unit of FIG. 15 is designated by the SS spread unit 3.
LPF 6I and LPF 6Q of FIG. 15 are low pass filters which apply the same process as that of the LPF 6 of FIG. 10 to the I and Q components. For simplicity, in the description of FIG. 15, LPF 6 designated both the LPF 6I and the LPF 6Q.
In other circuits of FIGS. 15 and 16, those circuits which function in the same way as those FIGS. 10 and 11 are designated by the same numerals. The operations of the respective circuits are identical to those of FIGS. 10 and 11 except the above points.
When the information of 1.5 Mbps is transmitted by the transmitter replaced by the delayed detection quadrature phase shift keying (DQPSK) system, the usable chip length of the PN code is not larger than 13 Mcps/1.5 Mbps.times.2 phases=17 chips.
For the 17-chip length, unlike the 50-chip length described above, the search by the computer may be possible because the chip length is short, but when the 15-chip PN code which is the maximum usable channel code is used as it is used for the wireless LAN, the chip rate is 1.5 Mbps.times.15 chips/2 phases=11.25 Mcps.
The band width of the main robe of the transmission signal is double of that, that is, 22.5 MHz&lt;26 MHz.
When two channels of information codes of motion pictures or semi-motion pictures are simultaneously received at the center in accordance with the second demand described above, it is necessary to make the reception levels of the two channels equal in order to obtain the images of the same image quality for the two channels.
However, when the conventional bandwidth utilization scheme as shown in FIG. 13A is used, the C/N is as low as 0 dB. Moreover, with the conditions above, the margin for the band width of 26 MHz of the transmission band is as narrow as 3.5 MHz (=26 MHz-22.5 MHz). For this reason, even if the main robes of the two channels are separated fully across the opposite sides of the transmission band B as shown in FIG. 13 B, the main robes of the two channels have a large overlap as shown in FIG. 13B, thereby reducing the C/N in simultaneous transmission of the two channels as almost equal to that in FIG. 13A. In order to attain an error rate no larger than approximately 1/10.sup.8 solely by the DQPSK system, the received signal having the C/N of no less than approximately 17 dB is theoretically needed. Accordingly, in order to simultaneously receive the signals of the two channels of 1.5 Mbps, a spread process gain of at least 17 dB is needed to fill the difference between the C/Ns. However, with the 15-chip length, a spread process gain of only approximately 12 dB is attained and the simultaneous transmission of the two channels of 1.5 Mbps by the second demand is not attained.